Semiconductor device having a junction portion contacting a Schottky metal

ABSTRACT

A semiconductor device according to the present invention includes a first conductive-type SiC semiconductor layer, and a Schottky metal, comprising molybdenum and having a thickness of 10 nm to 150 nm, that contacts the surface of the SiC semiconductor layer. The junction of the SiC semiconductor layer to the Schottky metal has a planar structure, or a structure with recesses and protrusions of equal to or less than 5 nm.

TECHNICAL FIELD

The present invention relates to a semiconductor device provided with aSchottky barrier diode made of SiC and a method of manufacturing thesame.

BACKGROUND ART

Conventionally, attention is paid to a semiconductor power device usedmainly for a system in various types of fields of power electronics suchas a motor control system and a power conversion system. As asemiconductor power device, an SiC Schottky barrier diode is well-known(for example, Patent Documents 1 and 2).

CITATION LIST Patent Literature

Patent Document 1: Japanese Patent Application Publication No.2005-79339

Patent Document 2: Japanese Patent Application Publication No. 2011-9797

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a semiconductor devicecapable of reducing forward voltage while suppressing a reverse leakagecurrent to a comparable level as in the conventional technology anddecreasing a variation in the reverse leakage current, and to provide amethod of manufacturing the same.

Solution to Problem

The semiconductor device of the present invention includes a firstconductive-type SiC semiconductor layer, and a Schottky metal being madeof molybdenum contacting a surface of the SiC semiconductor layer andhaving a thickness of 10 nm to 150 nm, in which the SiC semiconductorlayer has a first junction portion contacting the Schottky metal and thefirst junction portion is a flat structure or a structure havingunevenness of 5 nm or less.

According to the arrangement, the first junction portion of the SiCsemiconductor layer to the Schottky metal is a flat structure or astructure having unevenness of 5 nm or less. This reduces forwardvoltage while suppressing a reverse leakage current to a comparablelevel as in the conventional technology.

Further, in this structure, a thickness of the Schottky metal made ofmolybdenum is 10 nm to 150 nm, and thus, the stress applied to the SiCsemiconductor layer from the Schottky metal can be alleviated and avariation in the stress can be decreased. Thus, when the semiconductordevice of the present invention is mass-produced, it is possible todecrease a variation in the reverse leakage current. As a result, it ispossible to stably supply a semiconductor device having quality in whichthe reverse leakage current stays within a constant range. When thethickness of the Schottky metal is 10 nm to 100 nm, it is possible tofurther decrease the variation in the reverse leakage current.

It is preferable that the Schottky metal has a single crystallinestructure of which the crystalline interface is not exposed in avertical cross section. According to the arrangement, it is possible tomake uniform a characteristic of the entire Schottky metal.

It is preferable that the semiconductor device includes an anodeelectrode formed on the Schottky metal, and the anode electrode includesa second junction portion made of a titanium layer contacting theSchottky metal. In that case, the anode electrode may include analuminum layer formed on the titanium layer.

It is preferable that the semiconductor device includes a nickel contactlayer contacting a back surface of the SiC semiconductor layer.

The semiconductor device may include a cathode electrode including atitanium layer formed on the nickel contact layer. In that case, analloy layer may be further formed which contains titanium and carbonbetween the nickel contact layer and the cathode electrode.

The semiconductor device may further include a carbon layer formed onthe nickel contact layer.

The semiconductor device may include a second conductive-type guard ringformed to surround the first junction portion. In that case, the SiCsemiconductor layer may be made of n-type SiC and the guard ring may bemade of p-type SiC.

It is preferable that the guard ring is formed to extend outward withrespect to an outer circumferential edge of the Schottky metal.

When a load connected to the semiconductor device is inductive, if acurrent passing through the load is blocked, then counter-electromotiveforce generated to the load. Resulting from the counter-electromotiveforce, reverse voltage in which the anode side is positive may applybetween an anode and a cathode. In such a case, it is possible torelatively decrease a resistance value of the guard ring, and thus, itis possible to suppress heat generated by the current passing within theguard ring. As a result, it is possible to prevent a device from beingthermally destroyed. That is, it is possible to improve an inductiveload resistance (L load resistance).

Further, it is preferable that when the semiconductor device includes afield insulating film formed on a surface of the SiC semiconductorlayer, the field insulating film formed therein with an opening throughwhich the first junction portion and an inner peripheral portion of theguard ring are selectively exposed, the Schottky metal is joined to theSiC semiconductor layer within the opening and rides on the fieldinsulating film by a riding amount of 10 μm to 60 μm from acircumferential edge of the opening.

According to the arrangement, when the reverse voltage is appliedbetween the anode and the cathode as described above, it is possible toshorten a distance over which a current passes within the guard ring,and thus, it is possible to suppress heat generated by the current. As aresult, it is possible to prevent a device from being thermallydestroyed.

Therefore, when a dopant concentration of the guard ring and the ridingamount on the field insulating film in the Schottky metal are combined,it is possible to realize an excellent inductive load resistance (L loadresistance).

The Schottky metal may be formed so that an outer circumferential edgethereof contacts the guard ring.

A method of manufacturing a semiconductor device according to thepresent invention includes a step of forming a Schottky metal made ofmolybdenum having a thickness of 10 nm to 150 nm, on a surface of afirst conductive-type SiC semiconductor layer, and a step of performinga heat treatment on the Schottky metal in a state where the surface ofthe Schottky metal is exposed so that a first junction portion with theSchottky metal in the SiC semiconductor layer is made a flat structureor a structure having unevenness of 5 nm or less.

According to the method, the first junction portion of the SiCsemiconductor layer to the Schottky metal is made a flat structure or astructure having unevenness of 5 nm or less. This provides asemiconductor device capable of reducing forward voltage whilesuppressing a reverse leakage current to a comparable level as in theconventional technology.

Further, in this structure, a thickness of the Schottky metal made ofmolybdenum is 10 nm to 150 nm, and thus, the stress applied to the SiCsemiconductor layer from the Schottky metal can be alleviated and avariation in the stress can be decreased. Thus, when the semiconductordevice obtained by the method is mass-produced, it is possible todecrease a variation in the reverse leakage current. As a result, it ispossible to stably supply a semiconductor device having quality in whichthe reverse leakage current stays within a constant range.

It is preferable that the step of performing a heat treatment on the SiCsemiconductor layer is executed in an atmosphere where oxygen is notpresent. Specifically, it is preferable that the step of performing aheat treatment on the SiC semiconductor layer is executed in a nitrogenatmosphere. In that case, it is preferable that the step of performing aheat treatment on the SiC semiconductor layer is executed in aresistance heat furnace.

According to these methods, it is possible to prevent an oxidation ofthe Schottky metal (molybdenum) during the heat treatment anddeterioration of a surface portion of the Schottky metal into amolybdenum oxide.

It is preferable that the method of manufacturing a semiconductor deviceincludes a step of forming an anode electrode on the Schottky metal, andin the step of forming the anode electrode, a titanium layer is formedso as to contact the Schottky metal. In that case, the step of formingthe anode electrode may include a step of forming an aluminum layer soas to contact the titanium layer.

Further, it is preferable that the method of manufacturing asemiconductor device includes a step of forming a nickel contact layeron a back surface of the SiC semiconductor layer before the formation ofthe Schottky metal and performing a heat treatment on the nickel contactlayer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of a semiconductor device according to aembodiment of the present invention.

FIG. 2 is a cross-sectional view taken along a cutting plane line II-IIin FIG.1.

FIG. 3 is an enlarged view of a portion within a broken line circle inFIG. 2.

FIG. 4 is a flowchart for describing one example of a process ofmanufacturing the semiconductor device.

FIG. 5 is a view showing a modified embodiment of the semiconductordevice in FIG. 1.

FIG. 6 is a view showing a modified embodiment of the semiconductordevice in FIG. 1.

FIG. 7 is a view showing a modified embodiment of the semiconductordevice in FIG. 1.

FIG. 8 is a TEM image of a Schottky interface in a Reference Example 1.

FIG. 9 is a TEM image of a Schottky interface in Comparative Example 1.

FIG. 10 is a correlation diagram between Vf and Ir of Example 1 andComparative Example 1, respectively.

FIG. 11 shows If-Vf curves (Ta=25° C.) of Example 1 and ComparativeExample 1, respectively.

FIG. 12 shows If-Vf curves (Ta=125° C.) of Example 1 and ComparativeExample 1, respectively.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will hereinafter be described indetail with reference to the accompanying drawings.

FIG. 1 is a plan view of a semiconductor device according to aembodiment of the present invention. FIG. 2 is a cross-sectional viewtaken along a cutting plane line II-II in FIG.1. FIG. 3 is an enlargedview of a portion within a broken line circle in FIG. 2.

A semiconductor device 1 includes an element in which SiC is adopted,and is of a squared chip shape in a plan view, for example. Thesemiconductor device 1 may be of a rectangular shape in a plan view. Thesize of the semiconductor device 1 has 0.5 mm to 20 mm in the respectivevertical and horizontal lengths in the sheet of FIG. 1. That is, thechip size of the semiconductor device 1 is 0.5 mm square to 20 mmsquare, for example.

The surface of the semiconductor device 1 is divided by an annular guardring 2 into an active region 3 inside the guard ring 2 and an outercircumferential region 4 outside the guard ring 2. The guard ring 2 is asemiconductor layer containing a p-type dopant, for example. As thedopant to be contained, B (boron), Al (aluminum), Ar (argon), etc., maybe used. The depth of the guard ring 2 may be about 100 nm to 1000 nm.

With reference to FIG. 2, the semiconductor device 1 includes asubstrate 5 made of n⁺-type SiC and a drift layer 6 made of n⁻-type SiClaminated on a surface 5A of the substrate 5. In the embodiment, thesubstrate 5 and the drift layer 6 are shown as one example of the SiCsemiconductor layer of the present invention.

The thickness of the substrate 5 may be 50 μm to 600 μm, and thethickness of the drift layer 6 thereon may be 3 μm to 100 μm. As ann-type dopant contained in the substrate 5 and the drift layer 6, N(nitrogen), P (phosphorus), As (arsenic), etc., may be used. As for arelationship in dopant concentration between the substrate 5 and thedrift layer 6, the dopant concentration of the substrate 5 is relativelyhigher, and the dopant concentration of the drift layer 6 is relativelylower than that of the substrate 5. Specifically, the dopantconcentration of the substrate 5 may be 1×10¹⁸ to 1×10²⁰ cm⁻³, and thedopant concentration of the drift layer 6 may be 5×10¹⁴ to 5×10¹⁶ cm⁻³.

On a back surface 5B ((000-1) C plane, for example) of the substrate 5,a nickel (Ni) contact layer 7 is formed to cover the entire back surface5B. On the nickel contact layer 7, a cathode electrode 8 is formed. Thenickel contact layer 7 is made of a nickel containing metal forming anohmic j unction with the substrate 5. Such a metal may include a nickelsilicide layer, for example. In the cathode electrode 8, a structure(Ti/Ni/Au/Ag) is formed in which titanium (Ti), nickel (Ni), gold (Au),and silver (Ag) are laminated in order from the nickel contact layer 7side, for example, and an Ag layer is exposed to the topmost surface.

On a surface 6A ((0001) Si plane, for example) of the drift layer 6, afield insulating film 10 is formed which has a contact hole 9 throughwhich one portion of the drift layer 6, as the active region 3, isexposed and covers the outer circumferential region 4 surrounding theactive region 3. The field insulating film 10 may be arranged by SiO₂(silicon oxide) for example. A film thickness of the field insulatingfilm 10 may be 0.5 μm to 3 μm.

On the field insulating film 10, Schottky metal 11 and an anodeelectrode 12 are laminated.

The Schottky metal 11 contacts, via the contact hole 9, the surface 6Aof the drift layer 6, and forms a Schottky barrier with the drift layer6. Specifically, the Schottky metal 11 is made of molybdenum (Mo), andhas a thickness of 10 nm to 150 nm. The Schottky metal 11 is embedded inthe contact hole 9 and rides on the field insulating film 10 to cover acircumferential edge portion of the contact hole 9 in the fieldinsulating film 10 from above. More specifically, the Schottky metal 11preferably rides on the field insulating film 10 so that the guard ring2 extends (projects) outward with respect to an outer circumferentialedge 19 of the Schottky metal 11. In order that the guard ring 2 isprojected outward, for example, a width W (riding amount) from acircumferential edge of the contact hole 9 of a portion that rides onthe field insulating film 10 (riding portion 18) of the Schottky metal11 to the outer circumferential edge 19 preferably is 10 μm to 60 μm. Itis noted that in the embodiment, the circumferential edge of the contacthole 9 indicates a position at which the thickness of the fieldinsulating film 10 is 0 (zero). Therefore, for example, when the contacthole 9 is formed in a tapered shape in which the diameter is narrowerfrom the upper end to the lower end, the width W is measured from thelower end of the circumferential edge of the contact hole 9.

The Schottky metal 11 is relatively thin, that is, 10 nm to 150 nm, andtherefore, in the Schottky metal 11, it is possible to decrease a stepbetween an upper portion that rides on the field insulating film 10 anda lower portion contacting the surface 6A of the drift layer 6. Thisdecreases the step in the topmost surface of the anode electrode 12, andtherefore, it is possible to easily join a bonding wire to the topmostsurface.

The Schottky metal 11 may have a single crystalline structure of whichthe crystalline interface is not exposed in a vertical cross section.Whether or not the Schottky metal 11 is of single crystalline structurecan be confirmed by photographing and observing an image of a crosssection of the Schottky metal 11 by using TEM (Transmission ElectronMicroscope), for example. With the arrangement, it is possible to makeuniform a characteristic of the entire Schottky metal 11.

As shown in FIG. 3 here, when an uneven structure 13 is formed in ajunction portion 61 (one portion of the surface 6A) of the drift layer 6to the Schottky metal 11, a height H₁ of the uneven structure 13 is 5 nmor less. As in FIG. 3, when a plurality of recessed portions are formedin the uneven structure 13, the height H₁ of the uneven structure 13 mayadopt a depth at the deepest recessed portion. It is noted that theembodiment shows an example where the uneven structure 13 is formed inthe junction portion 61, and the junction portion 61 of thesemiconductor device 1 may be a flat structure where the unevenness isscarcely present.

The anode electrode 12 may be of a two-layered structure including atitanium layer 121 formed on the Schottky metal 11 and an aluminum layer122 formed on the titanium layer 121. The anode electrode 12 is aportion which is exposed to the topmost surface of the semiconductordevice 1 and to which a bonding wire, etc., are joined. Similar to theSchottky metal 11, the anode electrode 12 rides on the field insulatingfilm 10 to cover a circumferential edge portion of the contact hole 9 inthe field insulating film 10 from above. Preferably, the titanium layer121 has a thickness of 70 nm to 230 nm, and the aluminum layer 122 has athickness of 3.2 μm to 5.2 μm (4.2 μm, for example). More particularly,the titanium layer 121 may be of a two-layered structure including alower layer, that is, Ti, and an upper layer, that is, TiN. At thistime, a thickness of Ti is 10 nm to 40 nm (25 nm, for example), and athickness of TiN is 60 nm to 190 nm (130 nm, for example).

The guard ring 2 dividing the drift layer 6 into the active region 3 andthe outer circumferential region 4 is formed along the profile of thecontact hole 9 to cross over the inside and outside of the contact hole9 in the field insulating film 10 (to cross over the active region 3 andthe outer circumferential region 4). Therefore, the guard ring 2 has aninside portion 21 (inner peripheral portion) that projects inward of thecontact hole 9 and contacts a terminal end portion of the Schottky metal11 within the contact hole 9, and an outside portion 22 that projectsoutward of the contact hole 9 and faces the Schottky metal 11 with thecircumferential edge portion of the field insulating film 10 beinginterposed therebetween.

On the topmost surface of the semiconductor device 1, a surfaceprotective film 14 is formed. At a central portion of the surfaceprotective film 14, an opening 15 is formed through which the anodeelectrode 12 is exposed. The bonding wire is joined, via the opening 15,to the anode electrode 12. The surface protective film 14 may be of atwo-layered structure including a silicon nitride (SiN) film 141 formedon the anode electrode 12 and a polymide film 142 formed on the siliconnitride film 141. Preferably, the silicon nitride film 141 has athickness of 800 nm to 2400 nm (1600 nm, for example), and the polymidefilm 142 has a thickness of 5 μm to 14 μm (9 μm, for example).

When the semiconductor device 1 is in a forward bias state wherepositive voltage is applied to the anode electrode 12 and negativevoltage is applied to the cathode electrode 8, an electron (carrier)moves from the cathode electrode 8 to the anode electrode 12 via theactive region 3 in the drift layer 6, and as a result, an electriccurrent passes. Thus, the semiconductor device 1 (Schottky barrierdiode) operates.

According to the semiconductor device 1, the junction portion 61 of thedrift layer 6 to the Schottky metal 11 is flat or an uneven structure 13of 5 nm or less. Thus, it is possible to reduce a forward voltageirrespective of a use environment (ambient temperature, etc.) whilesuppressing a leak current (reverse leakage current) passing in areverse bias state to a comparable level as in the conventionaltechnology.

Further, in this structure, the thickness of the Schottky metal 11 madeof molybdenum is 10 nm to 150 nm (100 nm, for example), and thus, thestress (compressive stress indicated by an arrow in FIG. 3, for example)applied to the drift layer 6 from the Schottky metal 11 can bealleviated and a variation in the stress can be decreased. Thus, whenthe semiconductor device 1 is mass-produced, it is possible to decreasea variation in the reverse leakage current. For example, a processcapability index Cpk may be 1.0 or more (preferably, 1.3 to 3.0). As aresult, it is possible to stably supply the semiconductor device 1 ofquality in which the reverse leakage current stays within a constantrange.

The Schottky metal 11 rides on the field insulating film 10 so that theguard ring 2 extends (projects) outward with respect to the outercircumferential edge 19 of the Schottky metal 11. When a load connectedto the semiconductor device 1 is inductive, if a current passing throughthe load is blocked, then counter-electromotive force is generated tothe load. Resulting from the counter-electromotive force, reversevoltage in which the anode side is positive may apply between an anodeand a cathode. In such a case, it is possible to relatively decrease aresistance value of the guard ring 2, and thus, it is possible toshorten a distance over which a current passes within the guard ring 2.Thus, it is possible to suppress heat generated by the current passingwithin the guard ring 2, and therefore, it is possible to prevent thedevice from thermally being destroyed. That is, it is possible toimprove an inductive load resistance (L load resistance) of thesemiconductor device 1.

FIG. 4 is a flowchart for describing one example of a process ofmanufacturing the semiconductor device 1.

First, on the surface 5A of the substrate 5, the drift layer 6 isepitaxially grown (step S1). Next, by a CVD (Chemical Vapor Deposition)method, for example, a mask is formed on the surface 6A of the driftlayer 6, and via the mask, an impurity is implanted toward the surface6A of the drift layer 6. Thereafter, a heat treatment is performed onthe drift layer 6, and the guard ring 2 is thereby formed selectively onthe surface portion of the drift layer 6 (step S2).

Next, by a thermal oxidation method or a CVD method, for example, thefield insulating film 10 that completely covers the guard ring 2 isformed on the surface 6A of the drift layer 6 (step S3). Next, by asputtering method, for example, the nickel contact layer 7 is formed onthe back surface 5B of the substrate 5. Thereafter, the substrate 5 isplaced in an electric furnace, in which the nickel contact layer 7 issubjected to a heat treatment at a predetermined first temperature (stepS4). It is preferable that the heat treatment on the nickel contactlayer 7 is performed in an induction heater of which the interior isadjusted to a nitrogen atmosphere, for example. Next, the fieldinsulating film 10 is patterned to form the contact hole 9, and theguard ring 2 is selectively exposed to within the contact hole 9 (stepS5).

Next, by a sputtering method, for example, on the entire surface 6A ofthe drift layer 6, the Schottky metal 11 made of molybdenum (Mo) havinga thickness of 10 nm to 150 nm is formed. Then, the substrate 5 isplaced in an electric furnace, and subjected to a heat treatment at apredetermined second temperature in a state where the surface of theSchottky metal 11 is exposed (step S6). The heat treatment in a statewhere the surface of the Schottky metal 11 is exposed means applying aheat treatment on the Schottky metal 11 when a protective cap such asmetal and a film is not formed on the surface of the Schottky metal 11.The heat treatment on the Schottky metal 11 preferably is performed, forexample, in a resistance heat furnace of which the interior is adjustedto an atmosphere where there is substantially no oxygen inside thefurnace (in the embodiment, a nitrogen atmosphere). If the heattreatment is performed under a nitrogen atmosphere, then the surfaceportion of the Schottky metal 11 is not deteriorated into molybdenumoxide due to an oxidation of the Schottky metal 11 (molybdenum) duringthe heat treatment. This eliminates a need for forming a protective capon the surface of the Schottky metal 11, and thus, it is possible toprevent the Schottky metal 11 from being raised by the thickness of theprotective cap. As a result, it is possible to maintain the thickness ofthe Schottky metal 11 to 10 nm to 150 nm.

Next, on the Schottky metal 11, the titanium layer 121 and the aluminumlayer 122 are laminated in order to form the anode electrode 12 (stepS7), and the surface protective film 14 is thereafter formed (step S8).

Finally, the cathode electrode 8 is formed on the nickel contact layer7, and the semiconductor device 1 shown in FIG. 1, etc., is therebyobtained.

Although the embodiments of the present invention have heretofore beendescribed, the present invention can be further embodied in other forms.

For example, the semiconductor device 1 may be embodied in a modifiedembodiment shown in FIG. 5 to FIG. 7.

In FIG. 5, between the nickel contact layer 7 and the cathode electrode8, a carbon layer 16 is formed. The carbon layer 16 is formed, duringthe formation of nickel silicide (nickel contact layer 7) as a result ofthe reaction of nickel deposited on the back surface 5B of the substrate5 with silicon in the substrate (SiC) 5 by the heat treatment in step S4in FIG. 4, when redundant carbon (C) not contributing to the reaction isdeposited on the surface of the nickel contact layer 7.

On the other hand, in FIG. 6, between the nickel contact layer 7 and thecathode electrode 8, an alloy layer 17 containing carbon is formed. Thealloy layer 17 is formed when the carbon (C) made redundant during theformation of the above-described nickel silicide layer and titanium (Ti)of the cathode electrode 8 are alloyed as a result of an electrodematerial (Ti/Ni/Au/Ag) for the cathode electrode 8 being deposited andthen subjected to a heat treatment, for example.

That is, FIG. 5 and FIG. 6 show between the nickel contact layer 7 andthe cathode electrode 8, a layer resulting from the redundant carbonduring the formation of the nickel silicide layer may be formed, andonly one of the carbon layer 16 and the alloy layer 17 shown in eachfigure may be formed and both of these layers may be laminated andformed.

In FIG. 7, the field insulating film 10 is omitted, and the entire guardring 2 is exposed to the surface 6A of the drift layer 6. A terminal endportion of the Schottky metal 11 riding on the field insulating film 10in FIG. 2 covers across the entire circumference of the inner peripheralportion of the guard ring 2 so that the guard ring 2 extends (projects)outward with respect to the outer circumferential edge 19 of theSchottky metal 11. Thus, the terminal end portion of the Schottky metal11 is joined to the inner peripheral portion of the guard ring 2.

For example, an arrangement obtained by inverting a conductive type ofeach semiconductor portion in the semiconductor device 1 may be adopted.For example, in the semiconductor device 1, the p-type portions may ben-type and the n-type portions may be p-type.

The nickel contact layer 7 may be subjected to a heat treatment in aresistance heat furnace and the Schottky metal 11 may be subjected to aheat treatment in an induction heater.

It is possible to incorporate the semiconductor device (semiconductorpower device) of the present invention into a power module used for aninverter circuit arranging a drive circuit for driving an electric motorutilized as a drive source for an electric vehicle (including a hybridcar), a train, and an industrial robot, etc. It is also possible toincorporate the semiconductor device of the present invention into apower module used for an inverter circuit that makes a conversion sothat power generated by a solar cell, a wind power generator, otherpower generators (in particular, a private power generator) iscoordinated with power of a commercially-available power supply.

It is possible to combine the characteristics understood from thedisclosure of the above-described embodiment even between differentembodiments. Further, it is possible to combine the constituentcomponents presented in each embodiment within the scope of the presentinvention.

The embodiments of the present invention are only a specific exampleused to clarify the technical content of the present invention, and thepresent invention should not be interpreted by limiting to thesespecific examples and the spirit and scope of the present invention arelimited only by the attached scope of claims.

The present application corresponds to Japanese Patent Application No.2012-129219 submitted on Jun. 6, 2012 to Japan Patent Office, the entiredisclosure of which is incorporated herein by reference.

EXAMPLES

Next, the present invention will be described on the basis of an exampleand a comparative example, however, the present invention shall not belimited to the following examples.

Example 1, Comparative Example 1, and Reference Example 1

According to a flowchart in FIG. 4, 12 (in SiC wafers) semiconductordevices 1 having a structure shown in FIG. 1 were manufactured (Example1). The thickness of the Schottky metal 11 was set to 100 nm.

On the other hand, 20 semiconductor devices were manufactured(Comparative Example 1) in much the same way as in Example 1 except thatthe Schottky metal 11 was subjected to a heat treatment in the sameprocess (oxygen atmosphere) as the nickel contact layer 7 in a statewhere the surface of the Schottky metal 11 (molybdenum) having athickness of 400 nm was protected with molybdenum nitride (MoN) having athickness of 200 nm. A semiconductor device arranged to have molybdenumnitride (MoN) having a thickness of 200 nm on the Schottky metal 11(molybdenum) having a thickness of 400 nm was manufactured (ReferenceExample 1) according to a flowchart in FIG. 4.

<Evaluation>

(1) TEM Image

A Schottky interface of the semiconductor devices obtained by theReference Example 1 and Comparative Example 1 were photographed by TEM.The obtained images are shown in FIG. 8 and FIG. 9.

As shown in FIG. 8, it was found that in the Reference Example 1, theSchottky interface (joined portion with the Schottky metal in SiC) was asmooth flat structure. It was also found that the molybdenum (Mo) was asingle crystalline structure in which the crystalline interface was notexposed. It is noted that Example 1 also had a similar structure.

On the other hand, as shown in FIG. 9, it was found that in ComparativeExample 1, an uneven structure including a plurality of recessedportions (darkish portions in FIG. 9) having a depth of about 20 nm wasformed at the Schottky interface. It was also found that the crystallineinterface appeared inside the molybdenum (Mo).

(2) Relationship Between Vf and Ir

Next, in each of Example 1 and Comparative Example 1, a relationshipbetween a forward voltage Vf (1 mA) necessary for passing forwardcurrent of 1 mA and a reverse leakage current Ir was examined. FIG. 10is a correlation diagram between Vf and Ir of Example 1 and ComparativeExample 1, respectively.

As shown in FIG. 10, it was found that in Example 1 and ComparativeExample 1, there was a conflicting relationship between Vf and Ir, andwhen the reverse leakage current Ir was suppressed to a comparablelevel, Vf could be reduced in Example 1. That is, in Example 1 where theSchottky interface was flat (having a smaller amount of surfaceroughness), it is possible to reduce the forward voltage whilesuppressing the reverse leakage current to a comparable level as inComparative Example 1.

(3) Vf-If Characteristic

Next, a Vf-If characteristic of each of Example 1 and ComparativeExample 1 was examined. FIG. 11 shows If-Vf curves (Ta=25° C.) ofExample 1 and Comparative Example 1, respectively. FIG. 12 shows If-Vfcurves (Ta=125° C.) of Example 1 and Comparative Example 1,respectively.

As shown in FIG. 11 and FIG. 12, it was found that in temperatureregions where the ambient temperature Ta was either 25° C. or 125° C.,it was possible to decrease the forward voltage Vf in Example 1 ascompared to Comparative Example 1.

(4) Variation in Reverse Leakage Current

The process capability index Cpk of the reverse leakage current in eachof Example 1 and Comparative Example 1 was examined. As a result, it wasrevealed that Example 1 having Cpk=1.82 had a smaller variation inreverse leakage current than the Reference Example 1 having Cpk=0.38.

REFERENCE SIGNS LIST

-   1 Semiconductor device-   2 Guard ring-   5 Substrate-   6 Drift layer-   6A Surface-   61 Junction portion-   7 Nickel contact layer-   11 Schottky metal-   12 Anode electrode-   121 Titanium layer-   122 Aluminum layer-   13 Uneven structure-   16 Carbon layer-   17 Alloy layer-   18 Riding portion-   19 Outer circumferential edge

The invention claimed is:
 1. A semiconductor device, comprising: a SiC semiconductor layer having a front surface and a rear surface; an anode electrode having a first anode electrode layer and a second anode electrode layer, the first and second anode electrode layer being formed on a Schottky layer to define a multi-layered structure that is in contact with the front surface of the SiC semiconductor layer; a cathode electrode formed on the rear surface of the SiC semiconductor layer, and a field insulating film formed on the front surface of the SiC semiconductor layer, the field insulating film having an opening through which the multi-layered structure is in contact with the front surface of the SiC semiconductor layer, wherein a Schottky junction is formed between the Schottky layer of the multi-layered structure and the front surface of the SiC semiconductor layer, recesses are formed in a Schottky junction portion of the SiC semiconductor layer and the recesses are formed repeatedly in a cross section, a part of the multi-layered structure is embedded in the recesses, the multi-layered structure includes the Schottky layer, the first anode electrode layer on the Schottky layer, and the second anode electrode layer on the first anode electrode layer, and the Schottky layer and the first anode electrode layer are each thinner than the second anode electrode layer both on the front surface of the SiC semiconductor layer inside the opening and on an upper surface of the field insulating film.
 2. The semiconductor device according to claim 1, wherein a thickness of the first anode electrode layer is between 60 nm to 190 nm.
 3. The semiconductor device according to claim 2, further comprising: a surface protection film covering a peripheral portion of the anode electrode and a part of the front surface of the SiC semiconductor layer.
 4. The semiconductor device according to claim 3, wherein the surface protection film has a two-layered structure including a silicon nitride film and a polyimide film formed on the silicon nitride film.
 5. The semiconductor device according to claim 1, further comprising: a guard ring formed in the SiC semiconductor layer such that the guard ring surrounds the Schottky junction portion between the Schottky layer of the multi-layered structure and the front surface of the SiC semiconductor layer.
 6. The semiconductor device according to claim 1, wherein the SiC semiconductor layer has a first conductive type.
 7. The semiconductor device according to claim 5, wherein, the Schottky junction portion and an inner peripheral portion of the guard ring are selectively exposed through the opening of the field insulating film.
 8. The semiconductor device according to claim 7, wherein a part of the front surface of the SiC semiconductor layer is exposed from a circumference of the field insulating film, and the semiconductor device includes a surface protection film covering a peripheral portion of the anode electrode and the exposed part of the front surface of the SiC semiconductor layer.
 9. The semiconductor device according to claim 1, wherein each recess has a depth shallower than 20 nm.
 10. The semiconductor device according to claim 1, further comprising: a guard ring formed in the SiC semiconductor layer such that the guard ring outwardly extends beyond an outer peripheral edge of the anode electrode.
 11. The semiconductor device according to claim 1, wherein the field insulating film has a thickness of 10 μm to 60 μm.
 12. The semiconductor device according to claim 1, further comprising: a surface protection film covering a peripheral portion of the anode electrode and a part of the front surface of the SiC semiconductor layer, wherein an edge portion of the surface protection film reaches a side end surface of the SiC semiconductor layer.
 13. The semiconductor device according to claim 1, wherein the recesses have a depth not greater than 20 nm and are irregularly arranged on the SiC semiconductor layer.
 14. A semiconductor device, comprising: a SiC semiconductor layer having a front surface and a rear surface; an anode electrode having a first anode electrode layer and a second anode electrode layer, the first and second anode electrode layer being formed on a Schottky layer to define a multi-layered structure that is in contact with the front surface of the SiC semiconductor layer; a surface protection film covering a peripheral portion of the anode electrode and a part of the front surface of the SiC semiconductor layer; and a cathode electrode formed on the rear surface of the SiC semiconductor layer, wherein a Schottky junction is formed between the Schottky layer of the multi-layered structure and the front surface of the SiC semiconductor layer, recesses are formed in a Schottky junction portion of the SiC semiconductor layer and the recesses are formed repeatedly in a cross section, a part of the multi-layered structure is embedded in the recesses, the multi-layered structure includes the Schottky layer, the first anode electrode layer on the Schottky layer, and the second anode electrode layer on the first anode electrode layer, and the recesses have a depth not greater than 5 nm and are irregularly arranged on the SiC semiconductor layer.
 15. The semiconductor device according to claim 14, wherein the first Schottky layer and the first anode electrode layer are each thinner than the second anode electrode layer.
 16. The semiconductor device according to claim 15, wherein a thickness of the first anode electrode layer is between 60 nm to 190 nm.
 17. The semiconductor device according to claim 16, wherein the surface protection film has a two-layered structure including a silicon nitride film and a polyimide film formed on the silicon nitride film.
 18. The semiconductor device according to claim 14, further comprising: a guard ring formed in the SiC semiconductor layer such that the guard ring surrounds the Schottky junction portion.
 19. The semiconductor device according to claim 18, further comprising: a field insulating film formed on the front surface of the SiC semiconductor layer, the field insulating film formed therein with an opening through which the Schottky junction portion and an inner peripheral portion of the guard ring are selectively exposed.
 20. The semiconductor device according to claim 19, wherein the part of the front surface of the SiC semiconductor layer is exposed from a circumference of the field insulating film.
 21. The semiconductor device according to claim 14, further comprising: a guard ring formed in the SiC semiconductor layer such that the guard ring outwardly extends beyond an outer peripheral edge of the anode electrode.
 22. The semiconductor device according to claim 14, further comprising: a field insulating film formed on the front surface of the SiC semiconductor layer, the field insulating film having a thickness of 10 μm to 60 μm.
 23. The semiconductor device according to claim 14, wherein an edge portion of the surface protection film reaches a side end surface of the SiC semiconductor layer. 